Induced pluripotent stem cells iPSCs are adult cells that have been genetically reprogrammed to an embryonic stem cell-like state by being forced to express genes and factors important for maintaining the defining properties of embryonic stem cells. Although these cells meet the defining criteria for pluripotent stem cells, it is not known if iPSCs and embryonic stem cells differ in clinically significant ways. Mouse iPSCs were first reported in 2006, and human iPSCs were first reported in late 2007. Mouse iPSCs demonstrate important characteristics of pluripotent stem cells, including expressing stem cell markers, forming tumors containing cells from all three germ layers, and being able to contribute to many different tissues when injected into mouse embryos at a very early stage in development. Human iPSCs also express stem cell markers and are capable of generating cells characteristic of all three germ layers.
Although additional research is needed, iPSCs are already useful tools for drug development and modeling of diseases, and scientists hope to use them in transplantation medicine. In the original protocol, viruses were used to introduce the reprogramming factors into adult cells. In animal studies, the virus used to introduce the stem cell factors sometimes causes cancers. Researchers have now developed non-viral delivery strategies that are believed to have less chance of causing cancers.
This breakthrough discovery of iPSCs has created a powerful new way to “de-differentiate” cells whose developmental fates had been previously assumed to be determined. In addition, tissues derived from iPSCs will be a nearly identical match to the cell donor and thus probably avoid rejection by the immune system. The iPSCs strategy creates pluripotent stem cells that, together with studies of other types of pluripotent stem cells, will help researchers learn how to reprogram cells to repair damaged tissues in the human body.
However, creating viable large arrays of iPSCs for high throughput drug screening is problematic due to their sensitivity to environmental factors, available nutrients, fabrication techniques, handling, contamination, and dehydration, as well as the cells immediate three dimensional macro structure environment and the precision equipment necessary to dispense nano liter quantities of molecules of interest to specific array locations.